PivotLabels (TopLabels and SideLabels collections)
Creates an additional row or column containing the totals for all columns or rows of the pivot.
Expression.AddTotals()
An expression that returns a PivotLabel object
This example shows how to total the top label columns called Product:
ActiveDocument.Sections["Pivot"].TopLabels["Product Id"].AddTotals()
This example shows how to add a total to the side label rows called Quarter:
ActiveDocument.Sections["Pivot"].SideLabels["Quarter"].AddTotals()